POROSITY GENERATION AND RESERVOIR POTENTIAL OF OULDBURRA FORMATION CARBONATES, OFFICER BASIN, SOUTH AUSTRALIA

Abstract

Porosity generation and reservoir potential of the early Cambrian Ouldburra Formation in the eastern Officer Basin is delineated by combining petrographical, petrophysical and sedimentological studies. The shallow marine Ouldburra Formation consists of carbonates, mixed carbonates and clastics, clastics and evaporites. Detailed analysis of more than 100 samples shows that dolomitisation resulted in substantial secondary porosity development within the carbonates. Secondary porosity has also been generated within the mixed siliciclastic-carbonate zone by carbonate matrix and grain dissolution as well as by dolomitisation. Prospective reservoir units correspond to highstand shallow marine facies where short periods of subaerial exposure resulted in diagenetic changes.

Sedimentary facies and rock character indicate that sabkha and brine reflux models are applied to dolomitisation within the Ouldburra Formation. Dolomite mainly occurs in two stages: common anhedral dolomites formed early by replacement of pre-existing limestone, and saddle dolomite and coarse crystalline dolomite formed during the late stages of burial diagenesis, associated with hydrocarbon shows. The dolomite reservoirs identified are ranked on the basis of their porosity distribution and texture into groups I to IV. Dolomites with rank I and II exhibit excellent to good reservoir characteristics respectively.

The Ouldburra Formation shows many depositional and diagenetic similarities to the Richfield Member of the Lucas Formation in the Michigan Basin of the USA. Substantial oil and gas production from middle Devonian shallow water to sabkha dolomites makes the Richfield Member an attractive reservoir analogue to the Ouldburra Formation.